Bridge network



J H MENNIE 2,607,827 5 BRIDGE NETWORK Filed Dec. 21, 1948 1' Aug. 19, 1952 FIG.

OSCIL LA TOP INVENTOR J.H.MNN/E AMPLIFIER D $AME POTENTIAL X AT BALANCE EQUAL AT BALANCE THESE VECTORS A TTORNEY Patented Aug. 19, 1952 BRIDGE NETWORK John H. Mennie, Livingston, N. L, assignor to Western Electric Company, Incorporated, New York, N.-Y.,' acorporation of New York "Application December 21, 1948, Serial No. 66,525

'--"I-his invention relates to bridgemeasuring i circuits, and it is-an object of this invention 'to =f-prdvide an A. C. impedance 'bridge which is simple -ln desi'gnand which gives' 'accurate measurements ov'er a 'wide frequency and impedance range.

In the conventional current type of A. Clim- *pedaniiebridge measuring circuit, a grounded -source of driving potential is applied across the -B 'and D corners-of the bridge and a null indi- '-'cator=-'is-'connected across the A and- C- corners through a -=double shielded transformer. Since Afl cj'meters'ai-e not sensitive enough toin'dicate' the-degree of' unbalance except at bridge -=v6nages high enough to be dangerous and im- "practicahsometype of amplification of the de- "gree' of unbalance isrequired. This is "usually done -by connecting an amplifier detector of the vacuum tube type to the bridge by means of a transformer.

ln" utilizing the transformer in the detecting -circi'iitjit-has been'foundthat the inherent ca- -pac'i'tive' effect presentin the transformer windir'i'gs presents a problem thathas to be overcome -be'fore the bridge circuit'will function accurately. fThe'di'stribution of the capacitive effect throughvout the windings makes it difiicult to neutralize --thisefiect, and the expedient has been universally -adopted of puttinga shield around each winding of the transformer. This serves the purpose-of :cencentrating this capacitive'effect as an inter- -;-shield capacitance so that itcan 'xbeeasily calculated and dealt with.

This intershield capacitance, which usually :has'a value of.=from 30- mmfs. to ZOO-mmfs is then-neutralized by-the insertion of a compen- -sating capacitance in the'bridge circuit, so that the bridge is=enabled to-function accurately. In some variations of bridge circuits, compensation is made-by-the insertion of inductances into the circuit. Still other circuits require the use of-two transformers instead of one.

It-is obvious that the necessity of compena'sating'this'type of A. C. bridge circuit to offset stlieieffect =of the high 'intershield capacitance i-greatly comp-licates th'ewdesign of these bridges.

The conipensation 'a-lso'has the definite effect of limiting'th'efrequency range and the impedance arrange at which the bridge can operate. Aniotlier-result that the compensation part of ithe-circuit often represents a major part of the roost :of the-entire equipment.

Applicant overcomes the problem presented'by :thje =intershield capacitance by a "design which "eliminates :the transformer normally used in the detector circuit. :In the principal en bodiment,

'8 Claims. (01. 175-183) -A and C-corners of the bridge.

2 two sides of a bridge are driven out ofphase with each other in any 'convenient'manner, as for example by means of a low impedance center tapped winding on a transformer. This type of circuit when provided with a low capacitance voltage dividing network'may be directly connected to a high impedance-detector which will indicate'a null balance for the same impedance relationships between bridge arms that are 'well known-for any standard bridge network. I

Other objects and features will be apparent from the following .detailed description when considered in "conjunction with the accompanying drawing in which:

Fig. 1 is a schematic wiring diagramof'the invention;

generally designated as 13. This transformer 13 has its secondary l4 tapped and connected to ground at its midpoint D. The opposite end terminals of the secondary M are designated as Band B. e

The other corners of the bridge are conventional and are given the standard designation as the A, C and D corners, the D corner being grounded. The four sides of the bridge, .AB,

BC, CD and AD, each contain .imped-ances 'standardly designated as 21, Z2, Z3 and-Z4, re-

spectively.

'A'suitable'lead 15 is'connected between the Another lead [6 is connected-between the midpoint X of lead 15 and-the -D cornerof the bridge. A capacitance' I1 is connected in thelead l5 between the points'A and X," and a capacitance 48 "is connected in the lead l5 between the-points-Xand C. These two capacitances-are -small, i-usua-lly about one or "two "mmfs. Theymustrclosely dielectrlc type 'and are insulated to ground. This "gives the necessary-'ri'gidity'and also prevents the variation in dielectric losses resulting from the use of lnsulat'ions' other than-air'from affecting the operation of the -.two 1 capacitances.

,Il'iis possible to useanyhigh impedance elements,

The balance principle of the proposed bridge can best be understood by means of the vector.

digaram shown in Fig. 2. With reference to this diagram: A j Vector BD=DB' (by design of Vector ADzDC (at balance) Therefore, by geometry:

llmg

By dividing out I1 and I2:

transformer) which is the standard bridge relation at balance.

The conventional bridge balance relationship may also be shownto apply to the.proposed,out-. of-phase system' by means of thejsimplified bridge circuit voltage diagram shown in Fig. In Fig.3 the following voltage relationship exists when'the bridge is balanced, that is, when the detector voltage E between 'X and D is zero:

En1=Enz=I1Z4 =IzZ3 E1=Ez (by design of transformer) It is apparent that:

. E1I1Z4=IiZ1 and 1 E2-I2Z3:IzZ2 Therefore:

By dividing Equation 2 by Equation 3, we get: Z1 Z2 f It is thus evident that any standard bridge network may be converted tothe proposed 'sys-- balanced, the difference in the opposing voltages and DC causes the detector to indicate the degree of unbalance. when the bridge is' in balance, the voltages AD'and DC are equal and opposite, and a null balance is produced in the g A .I I I, 1

' Because'of the small values of'capacitances l1 and-l8, the compensation needed on grounded bridge circuits in accordance with the invention bun ' "former" are exact 180 phase opposition over 4 is reduced to a point where it no longer complicates the design of the bridge, and to a point where the cost of such compensation becomes an insignificant factor. If the bridge is of the unity type, no compensation at all is needed.

Although the embodiment of the invention illustrated uses a transformer between the source of power and the bridge, the cofst'fof this transformer is small compared to the double shielded transformer used in the conventional type of bridge circuit.

Whendesigning a bridge based on the outof-phase principle, it is extremely important to be suresthat. the two voltages developed across the split secondary winding of the input transthedesired frequency range. It is also very im- H portanai except for unity ratio bridges, to keep Ithe' reactance and resistance components of the secondary winding as low as possible in proportion to the Z1 and Z2 arms in series with these windings. This condition may be met by.

a, highpermeabilityj type core with but few turns for; the secondary winding, and; with the two halves of, the secondary winding being closely coupled.v

An illustration lof the advantageous use of the invention in other types of; circuits is-shownby referencewto Fig. .4, :;which illustrates a crystal filter circuit utilizing the bridge described in the; present invention";. 'Aftypical application for such a filter circuit would be asgachannel filter, for a-carrier system, thecircuitserving as .apassv band to separate onet-channel from the .others.

In Fig. 4, the, conventional filter circuit capacir;

tances are designatedias;C. inductance as'L, resistance as R andthe four crystal bridge arms asY1,Yz,Ya and. Y4.:

By the use of the new bridge set up in, place of the standard crystal bridge, the capacitances to ground are lowered, resulting in increased range and performance at high frequencies. The characteristic attenuation-frequency curve for the circuit is the same with either type of" bridge. In addition, when the standard balanced to ground crystal filter circuit is used with an amplifier, it has to connect to the amplifier through a transformer, since the amplifier is grounded. The use of the described'bridge cir-' cuit, since it'is grounded, eliminates the need for this amplifier coupling transformer with a resultant saving in cost. 3

It is to be understood that the-above-described arrangements are simply illustrative of theapplication of the principles of the invention." Numerous other arrangements may be readily-f devised by those skilled in the art which willembody the principles of the invention andfall within the spirit and scope thereof.

What is claimed is:

1. In an A. C. bridge circuit, the combination of four impedance elements serially connected in a closed loop to form a bridge having two pairs of conjugate terminals, a connection grounding each terminal of one pair, means independent of said impedance elements for placing alternating RUBEN of conjugate terminals, a connection grounding each terminal of one pair, means connected in series with the impedance elements for placing alternating potentials of opposite phases between the other terminals and ground, two small equal impedances connected in series between the other terminals, and means for indicating the potential with respect to ground of the point of connection between the impedances.

3. In an A. C. bridge circuit, the combination of a transformer, four impedance elements serially connected in a closed loop to form a bridge having two pairs of conjugate terminals, with the secondary winding of the transformer forming a part of two adjacent arms and the midpoint of the winding comprising one of the bridge terminals, connections grounding both the secondary midpoint and the other terminal of its pair, means operating through said transformer for placing alternating potentials of opposite phase between the other terminals and ground, and a capacitance voltage dividing network including two equal capacitances connected across the other terminals.

4. In an A. C. bridge circuit, the combination of a transformer, four impedance elements serially connected in a closed loop to form a bridge having two pairs of conjugate terminals, with the secondary winding of the transformer forming a part only of the series impedance of each of two adjacent arms and the midpoint of the winding comprising one of the bridge terminals, connections grounding both the secondary midpoint and the other terminal of its pair, means operating through said transformer for placing alternating potentials of opposite phase between the other terminals and ground, a capacitance voltage di-' viding network including two equal capacitances connected across the other terminals, and a null detector connected between ground and the midpoint of said network.

5. The method of operating a four terminal A. C. bridge network having one opposite pair of terminals grounded to form two bridge sides, 7

said method comprising the steps of applying each of two alternating voltages equal in magnitude but opposite in phase serially in each of the two sides of the bridge, and establishing the midpoint in the potential gradient between the other opposite pair of terminals, and utilizing a change of the potential of the midpoint of the potential gradient produced by an unbalance in the network.

6. The method of operating a four terminal A. 0. bridge network having one opposite pair of terminals grounded to form two bridge sides, said method comprising the steps of applying alternating potentials equal in magnitude but opposite in phase to the two sides of the bridge, establishing the midpoint in the potential gradiout between the other opposite pair of terminals, and measuring the unbalance in the network in terms of the potential between the midpoint of the potential gradient and ground.

7. In an A. C. bridge circuit, the combination of four impedance elements serially connected in a closed loop to form a bridge having two pairs of conjugate terminals, a connection grounding each terminal of one pair, means for placing alternating potentials of opposite phases between the other terminals and ground, and means .for responding to an unbalance of said bridge, said means including two small fixed and equal impedances connected in series between the other terminals, and a load circuit connected between ground and the point of connection'between the impedances.

8. In an A. 0. bridge circuit the combination of a transformer, four impedance elements, means connecting the four impedance elements and the secondary winding of the transformer serially in a closed loop to form a bridge having two pairs of coniugate terminals with one of said elements in each arm of the bridge, the secondary winding of the transformer forming a part of two adjacent arms and the midpoint of the secondary winding comprising one of the bridge terminals, connections grounding both the secondary midpoint and the other terminal of its pair, means operating through said transformer for placing alternating potentials of opposite phases between the other terminals and ground, a capacitance voltage dividing network including two equal impedances connected across the other terminals, and a null detector connected between ground and the midpoint of said network.

JOHN H. MENNIE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,371,395 Keeling, Jr Mar. 13, 1945 2,405,999 Collar et a1. Aug. 20, 1946 2,424,677 Brownlee July 29, 1947 2,507,566 Frisbie May 16, 1950 

